Product modulation is a correlation technique, widely used in both (transmitter) modulation and (receiver) demodulation circuitry. In this technique, a first signal waveform is multiplied with a second signal waveform and the resulting signal is then filtered to remove unwanted energy. Such a system is illustrated in U.S. Pat. No. 3,387,220 issued on June 4, 1968 to A. Lender. In a transmitter the first signal waveform advantageously comprises a carrier wave and the second signal waveform comprises a modulating signal. In a receiver the first signal is an incoming modulated signal and the second is a local reference against which the first signal is compared. One known reference signal is a pure tone whose phase and frequency are carefully regulated. Another known reference signal is a delayed version of the first signal which is useful in making a differential comparison. The latter has the advantage of not having to be carefully regulated in phase and amplitude as was the pure reference tone.
When the delayed version of the first signal is used as the second signal it is important to carefully control the value of the delay. In an example case of detecting a frequency modulated signal, an interval of delay is selected such that at a particular frequency f.sub.c the delay is equivalent to a 90 degree phase shift. When the input signal at frequency f.sub.c is multiplied with that same frequency phase shifted by 90 degrees, the resulting output signal has a net dc value of zero volts. Frequencies above or below f.sub.c will have a net negative or positive value and are readily detected by simple filtering and polarity detection. When, however, the delay is such that at f.sub.c a 90 degree phase shift is not achieved, then when the multiplication is performed the resulting output signal will not have a zero volt net dc value. This dc offset is known as bias distortion and causes degraded performance. Furthermore, the effects of additive noise are increased when the phase shift at the central frequency is other than 90 degrees.
Conventional methods for compensating delay (phase shift) teach the insertion of a filter section having a particular delay at the frequency of interest. This method involves size and cost increases in the receiver and is shown in FIG. 1.9 of Applications of Digital Signal Processing, edited by Alan V. Oppenheim, copyright 1978 by Prentice-Hall Inc.
It is therefore an object of this invention to minimize bias distortion in correlation detectors without a substantial size and cost penalty for compensation apparatus.
It is another object of this invention to provide a delay compensation technique that is readily adapted to sampled data systems.